Most television systems, such as television receivers and monitors, which include a picture tube as a display device, also include a picture tube beam current limiting (BCL) arrangement, sometimes also known as an automatic beam limiting (ABL) arrangement. A BCL arrangement operates to prevent excessive beam currents which tend to cause spot defocussing or "blooming" and which may also damage the picture tube under extreme conditions. Typically, a BCL arrangement senses the average beam current drawn by the picture tube by sensing the "resupply" current drawn from a high voltage power supply for the picture tube and develops a BCL control signal representing the magnitude of the beam current. The BCL control signal is coupled in a feedback manner to one or both of contrast and brightness sections of the television system so as to reduce the beam current should the sensed beam current exceed a predetermined threshold. The contrast section controls the gain of the video channel coupled to the picture tube and therefore the peak-to-peak amplitude of video signals coupled to the picture tube. The brightness section controls the black level of video signals coupled to the picture tube.
Most BCL arrangements operate in a sequential manner to by reducing the contrast of reproduced images before reducing their brightness because contrast changes are subjectively less noticeably by viewers than are brightness changes. Nevertheless, noticeable brightness variations may be produced in otherwise well composed images. Such BCL induced brightness variations are especially pronounced in television systems, such as large screen and projection systems, which employ relatively high gain video channels in order to increase the average picture light output.